1. Field of the Invention
The present invention relates to a method for uplink pre-distortion for a Multi-Carrier Code Division Multiple Access (MC-CDMA) telecommunication system.
2. Description of the Related Art
MC-CDMA has been receiving widespread interest for wireless broadband multimedia applications. Multi-Carrier Code Division Multiple Access (MC-CDMA) combines OFDM (Orthogonal Frequency Division Multiplex) modulation and the CDMA multiple access technique. This multiple access technique was proposed for the first time by N. Yee et al. in the article entitled “Multicarrier CDMA in indoor wireless radio networks” which appeared in Proceedings of PMIC'93, Vol. 1, pages 109-113, 1993. The developments of this technique were reviewed by S. Hara et al. in the article entitled “Overview of Multicarrier CDMA” published in IEEE Communication Magazine, pages 126-133, December 1997.
Unlike DS-CDMA (Direct Spread Code Division Multiple Access), in which the signal of each user is multiplied in the time domain in order to spread its frequency spectrum, the signature here multiplies the signal in the frequency domain, each element of the signature multiplying the signal of a different sub-carrier.
In general, MC-CDMA combines the advantageous features of CDMA and OFDM, i.e. high spectral efficiency, multiple access capabilities, robustness in presence of frequency selective channels, high flexibility, narrow-band interference rejection, simple one-tap equalisation, etc.
More specifically, FIG. 1 illustrates the structure of an MC-CDMA transmitter for a given user i. We consider here the uplink, i.e. we suppose that the transmitter is located in the mobile terminal (denoted MT) of a user i. Let di(n) be the symbol to be transmitted from user i at time nT to the base station, where di(n) belongs to the modulation alphabet. The symbol di(n) is first multiplied at 110 by the a spreading sequence (and a scrambling sequence which is here omitted for the sake of clarity) denoted ci(t). The spreading sequence consists of N “chips”, each “chip” being of duration Tc, the total duration of the spreading sequence corresponding to a symbol period T. Without loss of generality, we assume otherwise specified in the following that a single spreading sequence is allocated to the user. In general, a user may be allocated one or a plurality of orthogonal spreading sequences (multi-code allocation) according to the data rate required. In order to mitigate intra-cell interference, the spreading allocated to different users are preferably chosen orthogonal.
The result of the multiplication of the symbol di(n), hereinafter simply denoted di by the elements of the spreading sequence gives N symbols multiplexed in 120 over a subset of N frequencies of an OFDM multiplex. In general the number N of frequencies of said subset is a sub-multiple of the number L of frequencies of the OFDM multiplex. We assume in the following that L=N and denote ci(l)=ci(lTc), l=0, . . . , L−1 the values of the spreading sequence elements for user i. The block of symbols multiplexed in 120 is then subjected to an inverse fast Fourier transformation (IFFT) in the module 130. In order to prevent intersymbol interference, a guard interval of length typically greater than the duration of the impulse response of the transmission channel, is added to the MC-CDMA symbol. This is achieved in practice by adding a prefix (denoted Δ) identical to the end of the said symbol. After being serialised in the parallel to serial converter 140, the MC-CDMA symbols are converted into an analogue signal which is then filtered and RF frequency up-converted (not shown) before being amplified in amplifier 150 and transmitted over the uplink transmission channel. The MC-CDMA method can essentially be regarded as a spreading in the spectral domain (before IFFT) followed by an OFDM modulation.
The signal Si(t) at time t which is supplied to the amplifier before being transmitted over the reverse link transmission channel can therefore be written, if we omit the prefix:
                                          S            i                    ⁡                      (            t            )                          =                                            d              i                        ⁢                                          ∑                                  l                  =                  0                                                  L                  -                  1                                            ⁢                                                          ⁢                                                                    c                    i                                    ⁡                                      (                    l                    )                                                  ⁢                                  exp                  ⁡                                      (                                                                  j                        ·                        2                                            ⁢                      π                      ⁢                                                                                          ⁢                                              f                        l                                            ⁢                      t                                        )                                                  ⁢                                                                  ⁢                for                ⁢                                                                  ⁢                nT                                              ≤          t          <                                    (                              n                +                1                            )                        ⁢            T                                              (        1        )            where fl=(l−L/2)/T, l=0, . . . , L−1 are the frequencies of the OFDM multiplex. More precisely, it should be understood that the transmitted signal is in fact Re(Si(t)exp(j2πF0t)) where Re(.) stands for the real part and F0 is the RF carrier frequency. In other words, Si(t) is the complex envelope of the transmitted signal.
An MC-CDMA receiver for a given user i has been illustrated schematically in FIG. 2. Since we consider the uplink, the receiver is located at the base station.
After baseband demodulation, the signal is sampled at the “chip” frequency and the samples belonging to the guard interval are eliminated (elimination not shown). The signal obtained can be written:
                              R          ⁡                      (            t            )                          =                                                            ∑                                  i                  =                  0                                                  K                  -                  1                                            ⁢                                                ∑                                      l                    =                    0                                                        L                    -                    1                                                  ⁢                                                                  ⁢                                                                            h                      i                                        ⁡                                          (                      l                      )                                                        ·                                                            c                      i                                        ⁡                                          (                      l                      )                                                        ·                                      d                    i                                    ·                                      exp                    ⁡                                          (                                                                        j                          ·                          2                                                ⁢                        π                        ⁢                                                                                                  ⁢                                                  f                          l                                                ⁢                        t                                            )                                                                                            +                                          b                ⁡                                  (                  t                  )                                            ⁢                                                          ⁢              for              ⁢                                                          ⁢              nT                                ≤          t          <                                    (                              n                +                1                            )                        ⁢            T                                              (        2        )            where t takes successive sampling time values, K is the number of users and hi(l) represents the response of the channel of the user i to the frequency of the subcarrier l of the MC-CDMA symbol transmitted at time n.T and where b(t) is the received noise.
The samples obtained by sampling the demodulated signal at the “chip” frequency are serial to parallel converted in 210 before undergoing an FFT in the module 220. The samples in the frequency domain, output from 220, are despread by the spreading sequence of user i. To do this, the samples of the frequency domain are multiplied by the coefficients ci*(l) (here in the multipliers 2300, . . . , 230L−1) and then added (in adder 240). The summation result is detected in 250 for supplying an estimated symbol {circumflex over (d)}i. Although not represented, the detection may comprise an error correction decoding like a Viterbi or a turbo-decoding which are known as such.
Furthermore, in MC-CDMA as in DS-CDMA, equalisation can be performed at the receiving side in order to compensate for the dispersive effects of the transmission channel. In MC-CDMA, the samples in the frequency domain are respectively multiplied with equalising coefficients qi(l), l=0, . . . , L−1 (here in 2300, . . . , 230L−1). However, in MC-CDMA in contrast to DS-CDMA, there is no simple equalisation method for an uplink channel because the estimation of an uplink channel appears very complex.
Indeed in MC-CDMA, this estimation must be performed before despreading, i.e. at the chip level, when the signal from the different users are still combined. In contrast, in DS-CDMA, this estimation is usually performed after despreading, i.e. at the symbol level, and therefore separately for each user.
In order to overcome the problem of channel estimation, it has been proposed to implement a pre-distortion at the transmitter side (i.e. in the mobile terminal, denoted MT), so that a simple demodulator could be used at the receiver side without needing to estimate the channel. The basic idea underlying pre-distortion is to exploit the reciprocity of the transmission channels (in TDD), that is the downlink channel estimation performed for the downlink demodulation is used as an estimation of the uplink channel. This implies both TDD-operation (same frequency band used for the uplink and downlink), and relatively low MT mobility, i.e. low Doppler frequency.
An MC-CDMA TDD-system with (downlink) pre-distortion has been described e.g. in the article of D. G. Jeong et al. entitled “Effects of channel estimation error in MC-CDMA/TDD systems” published in VTC 2000-Spring Tokyo, IEEE 51st, Vol. 3, pages 1773-1777. Pre-distortion is simply effected by multiplying each frequency component of the MC-CDMA symbol to be transmitted by the inverse of the channel response coefficient at said frequency, i.e. hi−1(l). However, contrary to what is put forward in the above mentioned paper such downlink pre-distortion is not possible since the base station (denoted BS) cannot send one common pre-distorted multi-user signal which would have been optimised for the different propagation downlink channels from the base station to the mobile terminals (hi−1(l) depends on i). This problem does not exist for the uplink transmission channels and one could think to apply this pre-distortion technique for the uplink. However, multiplying the frequency components by the coefficients hi−1(l) may lead to a very high transmitted power if the uplink transmission channel exhibits deep fades (i.e. hi(l) may be close to zero for some subcarriers l). This high transmitted power decreases in turn the battery autonomy and may significantly increase the interference towards adjacent cells.